1. Technical Field of the Invention
The present invention relates to a telecommunications network and, in particular, to the efficient management of channel resources within a digital mobile communications network.
2. Description of Related Art
The general name of the connection between a particular mobile station traveling within a particular cell area and the base transceiver station (BTS) providing radio coverage for that particular cell area is the "radio interface" or "air interface". Historically, the communications of information across the air interface between a base transceiver station (BTS) and a mobile station has employed, so-called, analog modulation techniques. For example, Frequency Division Multiple Access (FDMA) technology has been widely utilized to assign each mobile station to one of a plurality of the frequency channels associated with the current cell area to communicate with the serving BTS. More recently, however, digital modulation techniques have been used in order to enhance the spectrum efficiency with which the bandwidth allotted to mobile communications is used. As an illustration, the two techniques of time division multiple access (TDMA) and code division multiple access (CDMA) have been utilized to allow communications to proceed between a BTS and a plurality of different mobile stations on a relatively limited amount of radio frequency bandwidth. The Global System for Mobile (GSM) communications system, for example, utilizes the TDMA concept with one TDMA frame per carrier frequency channel to communicate between a mobile station and a BTS. One frame consists of eight time-slots (TS). Each time-slot of a TDMA frame on a single frequency channel is referred to as a physical channel. Accordingly, there are eight physical channels per carrier in the GSM system. Each physical channel of the GSM system can be compared with one single channel in an FDMA-system, where every user is connected to the system via one of the associated frequencies.
The utilization of TDMA technology requires that a great quantity and variety of information must be transmitted between the serving BTS and the mobile station over the limited physical channels. For example, control data, service request data, actual traffic data, supplementary data, etc., have to be communicated over the physical channels. As a result, in order to distinguish one type of data from another, different logical channels have been named and mapped (assigned) on to the available physical channels. For example, actual speech is sent on the logical channel named "traffic channel (TCH)" occupying one or more physical channels. Paging of a called party mobile station is performed over the logical "paging channel (PCH)". Furthermore, synchronization of a mobile station with a serving BTS is performed over the logical "synchronization channel (SCH)" which occupies one part of the physical channels. Accordingly, depending on the type of information being transmitted, different logical channels are utilized. Needless to say, if more physical channels are assigned to a particular logical channel, a lesser number of physical channels are available for the rest of the logical channels.
Because of the limited physical channel resources, mobile service providers are often faced with channel resource management and dimensioning problems. Because of the fact that congestion in SDCCH and/or TCH logical channels results in lost calls and unsuccessful call setups, the efficient management of the SDCCH and/or TCH is critical for providing reliable mobile service to mobile stations traveling within a serving coverage area.
One of the ways to better utilize channel resources is for the serving base transceiver station (BTS) to efficiently and effectively release no-longer needed logical channels. As an illustration, whenever a particular mobile station attempts to release a seized logical channel (e.g., TCH and/or SDCCH), a request signal instructing the serving BTS to release the logical channel is transmitted by the mobile station. Upon receiving the request signal, the serving BTS transmits an acknowledgment signal back to the requesting mobile station confirming the release. The mobile station then stops communicating over the released channel and the logical channel should become available for other mobile stations. However, due to radio interference and other terrestrial hindrances, the requesting mobile station sometimes fails to receive the transmitted acknowledgment signal. Not knowing whether the first release request was ever received by the serving BTS, the mobile station then re-attempts to release the channel by transmitting another release request signal to the serving BTS over the same logical channel. The mobile station repeats the above re-transmission until the expected acknowledgment signal is received or a prescribed time period has elapsed. Within the GSM phase 1 standard, in case no acknowledgment signal is received from the serving BTS, mobile stations are programmed to re-transmit a release request signal up to five times with a 235 ms interval. As a result, in the worst case, the mobile station repeatedly transmits five Disconnect signals over the same logical channel for a total of 1,175 ms (235.times.5) after determining that it no longer needs the channel.
Similarly, the serving BTS and/or BSC also has no way of determining whether the transmitted acknowledgment signal has been received by the requesting mobile station. The serving BTS, therefore, also does not know whether any additional Disconnect signals will be received over the same SDCCH from the requesting mobile station. As a result, even after properly receiving the first release request from the mobile station, the serving BTS and BSC must delay the release of the logical channel for possible re-transmission of the Disconnect signals by the unacknowledged mobile station for the 1,175 ms delay period.
Delay of 1,175 ms for each and every release of SDCCH and a corresponding time delay for TCH are inefficient and wasteful management of valuable logical channel resources. Accordingly, there is a need for a mechanism to minimize the channel release delay within a serving BTS.